The present invention relates to method and apparatus for judging a timing phase of a modem which is used in data communication for judging a timing phase from a reception signal of a communication line and, more particularly, to method and apparatus for judging a timing phase of a modem for judging the optimum timing phase.
In a single sampling type automatic equalizer which is used in the case where baud rate frequency doesn't satisfy a sampling theorem, an overlap on a frequency spectrum occurs. A degree of overlap on the spectrum is largely influenced by a timing phase and causes a deterioration of the performance of the automatic equalizer. Therefore, the timing phase is judged from the line reception signal and tap coefficients of a filter are controlled, thereby controlling so as to obtain the optimum phase.
However, in the construction to judge the timing phase by using a digital signal processor, when the number of divided portions of a phase plane is increased to improve a judging accuracy, a processing amount and a hardware amount of an ROM and the like increase. Therefore, it is demanded to judge the timing phase at a high precision without being influenced by the phase dividing number.
FIG. 1 shows a receiving section of a conventional data modem. A reception signal from a communication line is sampled to a digital value and given to a demodulating section 16 and is demodulated as a vector signal (x+jy) having a real component x and an imaginary component y indicative of a signal point of a phase plane. The demodulated vector signal is given to a phase control filter 18, a roll-off filter 20, an AGC circuit section 22, and an automatic equalizer 24. Further, the correct data signal point is judged by a judging section (not shown) and, after that, the vector signal is converted into the original transmission data. A timing extracting section 34 and a timing phase judging section 36 are provided on the branch side of an output of the phase control filter 18. The timing phase judging section 36 is constructed by a phase rotating section 130, a region judging section 132, and a judgement information forming section 134.
The reason why the timing phase is judged and controlled so as to have the optimum phase will now be described as follows. First, in case of using an automatic equalizer of the double sampling type, as shown in FIG. 2, there is a relation of EQU f=2f.sub.B
between a sampling frequency f and a baud rate frequency f.sub.B. In this case, the sampling theorem is satisfied and there is no overlap on the frequency spectrum. Therefore, a level fluctuation depending on the timing phase doesn't occur between two signals due to the overlap of the spectrum. On the other hand, in case of using an automatic equalizer of the single sampling type, as shown in FIG. 3, there is a relation of EQU f=f.sub.B
between the sampling frequency f and the baud rate frequency f.sub.B. In this case, the sampling theorem is not satisfied. Therefore, in case of the single sampling type automatic equalizer, an overlap on the frequency spectrum occurs. In the case where the overlap on the frequency spectrum occurs, a variation occurs in the level of the reception signal due to the timing phase. For instance, when considering an overlap of two tone signals, they are enhanced when a phase difference is equal to 0.degree.. When the phase difference is equal to 180.degree., they are set off and becomes zero. Therefore, in the case where the timing phase is worst, an infinite gain is required for the automatic equalizer and the performance of the equalizer deteriorates.
Therefore, with respect to the automatic equalizer of the single sampling type, as shown in FIG. 1, the timing phase judging section 36 is provided, the timing phase of the reception vector signal is judged, and tap coefficients are feedback controlled by the phase control filter 18 so as to obtain the optimum phase.
FIG. 4 is a flowchart showing a conventional timing phase judging process. For example, as shown in FIG. 5, the phase plane is divided into sixteen phase regions every angle of 22.5.degree., thereby judging region numbers 1 to 16 to which the reception vector signal belongs. First, in step S1, the real component x and the imaginary component y of the input vector signal are sampled. In step S2, a quadrant is discriminated to see if the vector signal belongs to either one of the first to fourth quadrants. In steps S3, S5, S7, and S9, the processing routine advances to either one of steps S4, S6, S8, and S10 in accordance with the result of the quadrant judgement in step S2. The number N of regions which are necessary to rotate the vector signal to the first quadrant. In this case, since the phase plane is divided into 16 regions every 22.5.degree.,
First quadrant: N=0 (not rotate)
Second quadrant: N=4 (22.5.degree..times.4=90.degree. rotation)
Third quadrant: N=8 (22.5.degree..times.8=180.degree. rotation)
Fourth quadrant: N=12 (22.5.degree..times.12=270.degree. rotation)
In the next step S11, the vector is moved to the first quadrant by calculating the absolute value of the real component x and the imaginary component y. Phase judging processes in steps S12, S14, and S16 are executed. In the phase judging process, when it is now assumed that the real component after the vector was moved to the first quadrant is set to X and the imaginary component is set to Y, in the case where the region into which the vector was moved belongs to the region No. 1, namely, when a phase angle .theta. of the vector moved to the first quadrant lies within a range of EQU 0.degree..ltoreq..theta..ltoreq.22.5.degree., EQU Y/X=tan .theta..ltoreq.tan 22.5.degree.=0.414213562 EQU Y-0.414213562.multidot.X.ltoreq.0
On the other hand, when the phase angle .theta. lies within a range of EQU 22.5.degree..ltoreq..theta..ltoreq.90.degree., EQU Y/X=tan .theta.&gt;tan 22.5.degree.=0.414213562 EQU Y-0.414213562.multidot.X&gt;0
Therefore, by the sign of EQU (Y-0.414213562.multidot.X),
it is possible to judge whether the region belongs to the region No. 1 or not. Such a phase judging process is the process in step S12. In step S12, (Y-0.414.multidot.X) is simply shown as an abbreviation.
Similarly, in step S14, a check is made to see if the region belongs to the region No. 2 of the first quadrant or not. In step S16, a check is made to see if the region belongs to the region No. 3 or not. The results of the phase judgements in steps S12, S14, and S16 are obtained as judgement region numbers N.sub.0 =1 to 4 in steps S13, S15, S17, and S18. Finally, in step S19, the region No. N.sub.0 obtained in either one of the steps S13, S15, S17, and S18 is added to the number N of regions which are necessary to the rotation to the first quadrant obtained in either one of the steps S4, S6, S8, and S10, so that the region number N to which the vector signal actually belongs can be calculated. For example, in case of the input vector signal of the phase angle of 240.degree., as a result of the quadrant judgment in step S2, the processing routine advances to step S7 and the third quadrant is discriminated and N=8 is obtained. Due to the movement to the first quadrant, the condition in step S16 is satisfied. Step S17 follows and N.sub.0 =3 is obtained. Finally, in step S19, EQU N=N+N.sub.0 =3+8=11
is obtained and the judgement result of the timing phase is obtained as region number N=11.
However, in such a conventional timing phase judgment, in the case where the dividing number of the phase plane shown in FIG. 5 is increased in accordance with the nth power (n is a natural number) of 2 in order to raise a judgment precision of the timing phase, there are problems such that the region data which is used for the comparison judgment of the timing phase increases and a data amount in the ROM increases and the processing time becomes long. For instance, when it is now assumed that the region was divided into 128 regions in order to improve the judgement precision as compared with the 16 divided regions in FIG. 4, the region is divided every 2.28125.degree.. Since the number of divided regions of the first quadrant is equal to 32, 31 kinds of coefficients K of (Y-K.multidot.X) indicative of the phase angle of one phase region in the first quadrant are needed. The ROM amount increases. To judge 32 regions, the comparing processes of 31 times are necessary in the longest case. There is a problem such that the processing time also becomes long and a hardware of a high-speed DSP and the like must be prepared.